In the most elementary form such sensors comprise a deformable support having the general form of a beam of rectangular cross-section secured in cantilever to a framework. Such deformable support which is provided so as to flex along a so-called sensing axis includes, on the one hand, a quartz resonator which forms detection means and which is directly fastened thereto in a mid-zone. On the other hand, the support includes a seismic mass fixed to the end opposite to that by which it is secured to the framework. Thus, under the action of a force such as an acceleration, the mass is displaced and deforms the support and the resonator coupled thereto. Such deformation brings about a variation in the vibration of the resonator which constitutes a signal representative of the magnitude of the applied force. Such signal is thereafter processed by an appropriate electronic circuit.
More specifically, this invention concerns such sensors comprising furthermore protection means intended to protect said detection means against accidental shocks capable of producing forces which present a component perpendicular to the sensing axis.
It is known in effect that sensors of this type are delicate precision measuring instruments, the detection means of which may be easily damaged because of their extreme fragility. Such damage occurs most often during manipulation of the sensor at the moment of its assembly for instance on a vehicle or during its replacement, indeed during examinations for checking the operation thereof. Such damage comes about in particular when the detection means undergo forces which are not parallel to their sensing axis. This is the case when, for instance, the mass undergoes an acceleration along the longitudinal axis of the support and the detection means are formed by a quartz resonator adapted for bending strain. Effectively, such an acceleration provokes an unforeseen traction on the detection means for which they are not adapted in a manner that they rapidly reach their breaking point and fail.
Such a deterioration of the detection means is undesirable and may have serious consequences since it is invisible from the outside and such sensors are generally intended to equip automotive vehicles or any other passenger transport machines in which they are supposed to furnish information having a direct incidence on the behaviour of essential elements of the vehicle and consequently on the security of the passengers.
There is known, in particular from patent document FR-A-2 490 828, a sensor forming an accelerometer comprising a structure protecting against shocks. According to this document, the case of the accelerometer is coated with a silicone rubber intended to absorb eventual accidental shocks.
Although such coating absorbs and dampens a portion of the shocks, it does not avoid transmission of acceleration beyond a certain threshold and thus does not constitute a particularly reliable protection system for the detection means. Furthermore, the case of such accelerometer may not be disassembled without ruining the protection layer so that servicing such accelerometer is a long and expensive operation.
Thus, the present invention has as purpose to overcome the disadvantages of the above-mentioned prior art by providing a measuring sensor equipped with protecting means for its detecting means, such protecting means being of simple construction and being effective whatever be the intensity level of a shock received by the sensor or of an acceleration for which a component is located out of the sensing axis of the sensor.